In the case of line drivers, it is known to use so-called clocked or class D line drivers, which have, in particular, the advantage of a high efficiency, such that the power consumption of a communications facility using the driver circuit can be reduced. The basic principle of a clocked line driver circuit is based on first converting an analog input signal of the driver circuit into a pulse-width modulated signal which can then be amplified in a particularly efficient manner. Lowpass filtering is then used to obtain the amplified output signal of the driver circuit from the amplified pulse-width modulated signal. One requirement for performing the pulse-width modulation is an appropriate clock signal, on which the pulse-width modulation is based. The clock signal can be supplied by means of, for example, an external oscillator.
A further possibility for generating the clock signal consists in providing the driver circuit with a feedback loop, which connects an output of the driver circuit to an input of a feedforward path of the driver circuit such that there is formed, via the feedback path and the feedforward path, a closed loop which renders possible an oscillating behaviour of the driver circuit itself. Furthermore, a feedforward filter in the feedforward path and a feedback filter in the feedback path typically ensure a sufficient linearity of the signal transmission.
In the case of more exacting linearity requirements, such as those that exist, for example, for DSL communications applications with higher data transmission rates, it is however advantageous to use higher-order active filters as feedforward filters. In this case, the driver circuit may exhibit an oscillatory behaviour at different frequencies, of which the only one frequency concerned is the frequency desired for the operation of the driver circuit. Furthermore, the available parameter range for the desired oscillatory behaviour of the driver circuit is reduced by production variations or changes in the external environment.
In view of the above, it would be advantageous to provide a self-oscillating driver circuit that may be controlled in such a way that oscillatory behaviour of the driver circuit at unwanted frequencies is prevented in an effective manner.